FOC?

[Forresterwoods]

#5372

I was wondering. If an arrow has sufficient spine, does not a heavy broadhead make that arrow…FOC? Just a thought.:?

[stykbo]

#40079

I’m of the understanding that spine is irrelevant when determining FoC? Maybe I’m wrong?

[David Petersen]

#40087

Spine, on first glance, is irrelevant to FOC, which is simply a measurement of the balance point of an arrow in re its linear center point. Where spine becomes deeply relevant to FOC is that the more weight you have up front, the greater the foc (again, a simple matter of balance point). And to hang a lot of weight up front, you need high spine. For example, I am now shooting low-50s bows so would use something like 55-60 spine for a 125-grain head, which would deliver only normal (teens) FOC at best. But for the 300-grain heads I prefer, I need 85# spine for that same little bow, which setup elevates me barely into EFOC category (20+ percent weight forward of linear center). So the trick is to find the lightest possible shaft that can deliver the necessary spine, which is why carbons are untouchable in the EFOC arena.

[Jason Wesbrock]

#40818

Forresterwoods wrote: I was wondering. If an arrow has sufficient spine, does not a heavy broadhead make that arrow…FOC? Just a thought.:?

Technically speaking, if your tip and insert weigh more than your nock and fletching, your arrow will balance front of center.

[lyagooshka]

#40868

J.Wesbrock wrote: Technically speaking, if your tip and insert weigh more than your nock and fletching, your arrow will balance front of center.

😆 😆 😆

Sorry, that one got me.

I was doing some calculating, and I am still having an issue. I am starting to think it’s me, but I figure I’d check. I have a 30″ 2016 aluminum arrow at 10.6 GPI (grains per inch). My fletchings are 5.8gr, so round to 18gr for 3. I have a 250gr broadhead and a 100gr insert. I tend not to follow directions well (and I don’t have my arrows in front of me) so I did not measure the shaft, but I figured I would use some math. Here’s what I got:

Total weight = 686gr (arrow = 318 {10.6*30} + 18 for fletching, 100 for insert and 250 for the broadhead)

Center of shaft is 15″ (30/2)

Center of weight would be the point at which both sides weigh 343gr. Here’s where it gets weird. Looking at the feather side, I would subtract the feathers from the [half] weight and get 325gr. That is how much shaft is on the non-broadhead side. So, divide that by the GPI and I get 30.66. Basically my FOC for this arrow is BEYOND the arrow.

HUH?????

So, other than my math being off, what am I missing? Keeping with arithmetic, I cannot find 15.66” on Dr. Ashby’s FOC chart.

Help!

Be well.

Alex

😀

[Jason Wesbrock]

#40879

Alex,

I plugged your data into Stu Miller’s dynamic spine calculator and it gave me a 685-grain arrow balancing at 24.2% FOC. Comparing my own arrows against the calculator, I’ve found it’s very close.

[lyagooshka]

#40921

I guess that’s where I get into trouble. My arrow dynamic spine and bow dynamic spine are NOWHERE close. If I decide to tweak the arrow to match the bow, I get an FOC about 10%, which is not acceptable at all according to the study here. I cannot tweak the bow (LB 52@28 or Hybrid {R/D} 50@28 ), so my only option is to be grossly mis-matched? Furthermore, I am getting a bow that is close to 80#. To have 350gr (250 + 100) up front, I cannot find a shaft that will handle it (according to the dynamic spine calculator) or an arrow that has acceptable FOC (>18%). So what am I doing wrong? Be well.

Alex

😕

[tombow]

#40996

I think we can get into trouble thinking that mathematical calculations and the numbers resulting are the be all end all of whether an arrow will fly well from YOUR BOW with YOU shooting it. I have a low 50’s bow like Mr. Dave but found that 80-85 spine woods were too stiff for me unless the shafts are full length, 31.5″ to BOP. I currently am shooting 29.5″ BOP 70-75 spines that are shooting awesome from MY bow with ME shooting it. What I am getting at is that each individual will likely have different shooting results because of how they draw, hold the bow and release.

Excellent arrow flight is the name of the game to me, especially when you start talking about arrow penetration. I can’t imagine that a High FOC arrow that flies poorly will out penetrate a lower FOC arrow that is flying straight, putting it’s weight firmly behind the broadhead at impact.

Forgive me if I am missing the point in what your saying, it’s just that I have used Stu’s calculator alot and have found that when I punch in numbers there I ended up with different spine numbers, Bow requirements vs. my spine numbers. But my arrows fly well irregardless of what the numbers say.

[lyagooshka]

#41009

Tombow wrote: Forgive me if I am missing the point in what your saying…

Tombow,

I agree completely with you (and Dave above where he says “So the trick is to find the lightest possible shaft that can deliver the necessary spine…”). the point I was making is that having a decent FOC (like may arrows at close to 25%) and having them balanced with the bow according to Stu’s chart if pretty darn difficult.

The calculator says “These two numbers should be nearly equal (within 2#). Fine tuning can then be done by adjusting the bow’s brace height.” My numbers are about 25# off. Then I am planning for the longbow I have on order and it looks like I will either have to use a steel rod for a shaft or give up a significant amount of FOC.

Using the calculator with the stats above on my 52# LB, I get arrow spine of (+/-) 22 and bow spine of 69.5. I am estimated at 158 fps and 38 ft/lbs. and an FOC just under 25%. Going to my new (WIP) bow gives me the same FOC, but bumps my speed to 192 fps and 56 fl/lbs. But my “spine must be within 2#” goes to 22 arrow and 97 bow. I understand that a good flying arrow can come out of a bow/shooter combo that is over/under “within 2#”, but such a big difference? I guess the numbers are just scaring me.

Going to what Dave said, I choose a 300 spine carbon arrow with all else being equal (for the 80# bow, I am using 78# as input). I still get 52# arrow and 97# bow. So my question is (I guess) how much can the spine suffer with FOC, and if it just depends on the shooter, why bother calculating it in the first place? Again, very new at this, and I actually want to get this right not just for me, but for the animal’s sake as well. Thank you all for your input. Be well.

Alex

😕

[David Petersen]

#41013

Alex — you are making FOC too difficult. You don’t need to use numbers at all, aside from two simple measurements: linear center point of shaft from back of head to bottom of nock … and balance point, which I find by balancing a shaft on the edge of a ruler turned on its side. Put a mark at each of these spots and measure the distance between them. Now go to the Ashby Library on this site and the FOC calculator chart, plug in the requested info and read your FOC. I can barely count my fingers, I’m that bad at math. But with the chart it’s a snap.

So far as the gentleman’s comment that FOC can’t be as important as perfect arrow flights … sounds like you’ve been reading Ashby. 😀 Perfect arrow flight comes first, as a wobbling arrow sucks at penetrating. Next comes arrow weight, then FOC, and of course you have to use a good “tissue dynamic ” (like aerodynamic but with muscle and bone rather than air) broadhead. It’s a combination of many factors but arrow flight is foremost and I’ve not heard anyone argue otherwise.

[lyagooshka]

#41052

Dave,

Thank you for the reply. I mean this in the most non-confrontational way imaginable, with all due respect and keeping in mind that I am a newbie: so what is the dynamic spine calculator used for? I have always been a numbers guy. I can explain myself in numbers 127.479 times better than with words 😛 . I plugged my arrow numbers into it, got an FOC that seems acceptable, but a spine that is WAY off (with the Tusker Concord Broadheads someone on the forum suggested 😉 ). I still understand the “get your arrow to fly perfect and worry about the rest after”, so is that what I should be doing with the spine calculator? If I am truly making this way harder than it should be, just tell me to go pound sand and I’ll go practice instead of number crunching. Thanks again to everyone for all your help and patience. It is truly appreciated. Be well.

Alex

😕 ❓ 😕 ❓ 😕

[David Petersen]

#41168

Alex — I haven’t a clue about the dynamic spine calculator and have never even used used the term, much less the calculator, but have heard it praised by many here. I’m sure it’s a good thing, but totally foreign to me, sorry. Here is how I set up my arrows, sort of by brail: Get a test set of arrows with no more than 5# steps in spine. Last summer Fletcher sent me a set of 9 Sitka spruce spined from high 70s to low 90s, knowing from experience that I was going to need a lot of spine to handle a 300-grain head on a wood shaft (53# bow). All were already cut to my preferred length of 29.5″ and fletched. It took only minutes to figure out I needed 85# spine, with a latitude of 3# either side. Knowing that, I then had Rick (Fletcher, the Feathered Shaft) make me up a dozen finished arrows and every one shoots like it was radar guided. I killed an elk, heart shot at 20 yards, with one of those this year. Then I talked to Hildebrand, the major source for SS shafts, and they were able to scrounge around and find another dozen raw tapered shafts that fit the bill, and I built them from there. They too are shooters, with FOC 21-23%. The only number that came into this entire process was 85. You could do the same thing with carbon or aluminum shafts. More ways than one to skin a cat, like Daddy used to say. In sum, start with the head weight you want to shoot and work from there.

[Raymond Coffman]

#41285

I find it very easy to determine my arrows FOC by simply following the good Doctors FOC chart contained within the Dr Ashby Library.

Just need a magic marker /tape measure/ ruler / and a calculator – I similar to Dave, havn’t used my college math in many yrs–

Scout

[stykbo]

#49959

I plugged your data into Stu Miller’s dynamic spine calculator ~J. Wesbrock

Where is this spine calculator? Being old school, I’d like to “play” with my data? Thank you.

[lyagooshka]

#50006

stykbo,

You can do a search on bing.com for “Stu Miller’s dynamic spine calculator”. It should be one of the top choices. Or I can just give you the link here: http://heilakka.com/stumiller/ . Oops! 😳 It is an Excel file. You can play online or download it. There is something very similar (I believe it is even credited to Mr. Miller) on 3RiversArchery.com ( http://www.3riversarchery.com/spinecalculator.asp ). It is the same exact thing, just in a different format, but the info is exactly the same. Hope this helps. Be well.

Alex

😀

[stykbo]

#50016

Thank you Mr. lyagooshka…

I did in fact do a search and found your first link but it would not take my info. No doubt because I did not know how to use it. The 3Rivers’ link is better. Thanks.

[lyagooshka]

#50033

stykbo wrote: but it would not take my info…

😳 Oops! That’s my fault. I meant you can download it there or use it online at 3 Rivers. Fy fault. Glad you were able to get to it. Be well.

Alex

😀

[stykbo]

#50079

I currently shoot wood arrows 55# spine, 3 x 5 feathers, 30″ with 125 grain snuffers. If I switch to 60# spine and 3 x 4 feathers I’ll have a perfect 51.8 Dynamic Spine. Not impressed with Arrow Energy readout though. But more often than not, I have had pass-throughs.

How are these “numbers” viewed in the world of “opinion”?

[Bruce Smithhammer]

#12389

I have yet to find a spine calculator that is oriented to, or useful for, higher FOC setups. So I don’t bother with them. I just start with a shaft that is in the ballpark spine range, a point weight that is roughly what I want, and keep tweaking until it flies right.

I’m shooting GT Trad 5575’s cut to 28.5″ with a 100gr. insert, a 225 gr. head and an additional 10gr. collar/washer. Flies great out of my #54 hybrid. FOC is 23.6%

[stykbo]

#12422

May I ask a question? WHY do we want such heavy broadheads 200-300 gr.)??

I am 65 yrs old, been bowhunting since 1968…back when Bob Swinehart was writing for “Archery World”??? I eventually learned and insisted on perfect arrow flight. Perfect is when you say it’s perfect. I have shot Zwickeys and Snuffers primarily (125-150 gr) and POC arrows. I will not shoot carbons. My bow is 50# @ 28″. I have 27″ draw. I almost always got pass throughs (I shoot close) and have not lost deer. Why would I now want to go to 200-300 gr broadheads?

[Bruce Smithhammer]

#12524

stykbo wrote: May I ask a question? WHY do we want such heavy broadheads 200-300 gr.)??

I am 65 yrs old, been bowhunting since 1968…back when Bob Swinehart was writing for “Archery World”??? I eventually learned and insisted on perfect arrow flight. Perfect is when you say it’s perfect. I have shot Zwickeys and Snuffers primarily (125-150 gr) and POC arrows. I will not shoot carbons. My bow is 50# @ 28″. I have 27″ draw. I almost always got pass throughs (I shoot close) and have not lost deer. Why would I now want to go to 200-300 gr broadheads?

I can only tell you why I like heavy broadheads, and the reasons are several;

1) The primary species I hunt are elk. I like to hedge my bets. A high-FOC setup increases momentum, and therefore penetration, particularly in less-than-ideal shots that might hit bone. I strive for perfect arrow flight, but that doesn’t mean I assume that every shot I take in real-world hunting circumstances is going to be perfect.

2) I find that high-FOC setups, when combined with the right shelf cut, are actually quite easy to tune.

3) Because I have yet to find any downside to increased FOC, and when combined with the benefits mentioned above, it’s a no-brainer, for me. While I tune my setup for large game, it works just fine on anything smaller. The opposite can’t always be said.

Everyone must come to their own conclusions. I just highly encourage people to come to conclusions after having experimented extensively with these ideas, rather than before.

[Ed Ashby]

#12761

DAVE P., TOMBOW AND SMITHHAMMER HAVE GIVEN LOTS OF GOOD ADVICE HERE. I’M A MATH GEEK TOO, BUT DON’T MAKE IT HARDER THAN IT NEEDS TO BE.

WOULD TYPE MORE BUT RIGHT HAND USELESS UNTIL I GET SURGERY ON TRAPPED ULNAR NERVE. WORKING LEFT HANDED, WITH JUST ONE TYPING FINGER.

ED

[tombow]

#16100

Good luck with the surgery Dr. Ed. OUCH!

And thanks for the penetration studies and information. You are the guru and we are all students learning from your information. Dang, sure is fun to discuss but I’d much rather test by shooting and find out what works best FOR ME. None of us is exactly the same and wouldn’t that be boring if that was the case??

[Author]

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